There are metal alloys, for example, some aluminum and titanium alloys, that display exceptional ductility when deformed under controlled conditions. They are susceptible to extensive deformation under relatively low shaping forces. Such alloys are characterized as being superplastic. The tensile ductility of superplastic metal alloys typically ranges from 200% to 1000% elongation.
Superplastic alloy sheets are formed by a variety of processes into articles of manufacture that are frequently of complex shape. These superplastic forming (SPF) processes are usually relatively slow, controlled deformation processes that yield complicated products. But an advantage of SPF processes is that they often permit the manufacture of large single parts that cannot be made by other processes such as sheet metal stamping. Sometimes a single SPF part can replace an assembly of several parts made from non-SPF materials and processes.
There is a good background description of practical superplastic metal alloys and SPF processes by C. H. Hamilton and A. K. Ghosh, entitled "Superplastic Sheet Forming" in Metals Handbook. Ninth Edition, Vol. 14, pages 852-868. In this text several suitably fine grained, superplastic aluminum and titanium alloys are described. Also described are a number of SPF processes and practices for forming superplastic materials. One practice that appears to be adaptable to forming relatively large sheets of relatively low cost superplastic aluminum alloys into automobile body panels or the like is stretch forming.
As described, stretch forming comprises gripping or clamping the flat sheet blank at its edges, heating the sheet to its SPF temperature and subjecting one side to the pressure of a suitable gas such as argon. The central unclasped portion of the sheet is stretched and plastically deformed into conformity with a shaping surface such as a die cavity surface. The term "blow forming" applies where the working gas is at super-atmospheric pressure (e.g., up to 690 to 3400 kPa or 100 psi to 500 psi). Vacuum forming describes the practice where air is evacuated from one side of the sheet and the applied pressure on the other side is limited to atmospheric pressure, about 15 psi. As stated, the sheet and tools are heated to a suitable SPF condition for the alloy. For SPF aluminum alloys, this temperature is typically in the range of 400.degree. C. to 550.degree. C. The rate of pressurization is controlled so the strain rates induced in the sheet being deformed are consistent with the required elongation for part forming. Suitable strain rates are usually 0.0001 to 0.01 s.sup.-1.
In stretch forming, a blank is tightly clamped at its edges between complementary surfaces of opposing die members. A schematic example is shown in FIG. 9, page 857 of the Hamilton et al article, supra. At least one of the die members has a cavity with a forming surface opposite one face of the sheet. The other die opposite the other face of the sheet forms a pressure chamber with the sheet as one wall to contain the working gas for the forming step. The dies and the sheet are maintained at an appropriate forming temperature. Electric resistance heating elements are located in press platens or sometimes embedded in ceramic or metal pressure plates located between the die members and the platens. A suitable pressurized gas such as argon is gradually introduced into the die chamber on one side of the sheet, and the hot, relatively ductile sheet is stretched at a suitable rate until it is permanently reshaped against the forming surface of the opposite die. During the deformation of the sheet, gas is vented from the forming die chamber.
The superplastic sheet employed in the SPF process is capable of undergoing appreciable elongation. However, since the sheet is clamped between the die members in a gas-tight seal, the only material available for the stretch forming is the area of the sheet within its clamped edges. Deformation of the sheet is seldom uniform, and excessive thinning of the sheet is likely in the more elongated regions. In the forming of pan-shaped articles, for example, it is often difficult to produce a tear-free product of reasonably uniform thickness across the part.
It is desired to adapt SPF practices to forming panels of complex shape for automotive applications. Light weight aluminum alloy sheets, for example, could be blow formed or vacuum formed into intricately shaped thin wall structures incorporating many subcomponents that would have required separate manufacture and assembly using less ductile aluminum alloys, for example, and conventional stamping practices. However, such intricate components must have reasonably uniform wall thickness and they must be free of tears and breaks. A robust process is required for high volume, low cost production of large stretch-formed parts. A process is required that can produce uniformly high quality parts in day-to-day manufacturing operations. Present SPF stretch form processes do not fill this need.